This invention relates to radar receivers, particularly to those for FMCW radar signals.
CW radar is generally used in preference to pulse radar, in order to maximise the power transmitted by the radar for a given peak power capability. FM or chirp enables the radar return signals to be related to the transmitted signals.
Referring to FIG. 1, a method of relating the phase of the radar return signals to that of the transmitted signals, in order to determine the range of targets, consists of replacing the fixed local oscillator which conventionally converts incoming r.f. radio signals to i.f. signals, by an oscillator which produces a sweep signal in synchronism with the transmitted signal. R.f. radar return signals received at the antenna 1 are mixed by mixer 2 with oscillations from an oscillator 3 which are mixed by mixer 4 with the r.f. sweep oscillations. Typical received and reference sweep signals of bandwidth B and period T are shown in FIG. 2. The corresponding output of the mixer 2, representing the difference between the frequencies of the received and reference signals, will be of the form shown in FIG. 3. The output will be, except in the region of the flybacks, a constant difference frequency .DELTA.f corresponding to the vertical displacement between received and reference sweep waveforms, since these sweeps are linear. This output .DELTA.f of course corresponds to a target of a particular range. The phase of the received sweep, relative to the reference sweep, for a target at a different range will be different, and the output of the mixer 2 will have a component at a different frequency. For a target at one particular range, the received sweep will be in phase with the reference sweep, and the difference frequency will be zero. For ranges greater or less than that range, the difference frequency will be positive or negative. It follows that the range of targets illuminated by the transmitted FMCW signal can be detected by a frequency analysis of the output of the mixer 2, and this is done by performing a Fourier Transform e.g. an FFT on a signal derived from the output of the mixer. A typical result of such an analysis is shown in FIG. 4. Each vertical line indicates the amplitude of the radar return for a range cell centered on the range corresponding to that line. The large central component corresponds to the received waveform being in phase with the reference wave form. The corresponding range is 2/T, multiplied by the velocity of light. The other components correspond to targets at different ranges.
It will be noted that, with this method, known as the de-ramping method, the components on each side of the zero frequency (dc) component are narrow and are therefore well defined because the difference frequency remains constant, except in the region of the flyback, since the reference and received sweeps are linear.
The method of FIG. 1 is subject to a number of disadvantages. In the case of an array of antennas, such as might be used in the case of high frequency (HF) radar, it would be necessary to provide reference sweeps identical in amplitude and synchronised in phase at the receiver for each antenna, which could be widely spaced. An analogue signal could be fed to each receiver, but the lengths of the distribution lengths would differ, and distortions could be introduced into the sweep signals. The sweep signal could be generated digitally and fed digitally to each receiver. In this case, the mixer 2 would have to be supplied with a fixed local oscillator and the de-ramping with the digital sweep signal would have to take place after conversion of the analogue i.f. signals to digital form.